Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever becomes absorbed in eating clay has helped against himself."
This Abdullah bin Marwan is majhul (unknown), and its meaning has been narrated through another chain that is also unknown.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19718
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever eats clay has himself helped in killing himself."
Abu Ahmad (Ibn Adi) said: I do not know anyone who narrates this from Suhayl bin Abi Salih except this Abd al-Malik, and he is majhul (unknown).
The Shaykh said: Even if this were authentic, it would not indicate prohibition; rather, there is dislike in that thing whose excess harms the body.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19719
Sufyan bin Abd al-Malik (may Allah have mercy on him) said: It was mentioned before Abdullah bin al-Mubarak (may Allah have mercy on him) that eating clay is unlawful, and he rejected it and said: If this were the saying of the Prophet (peace and blessings be upon him), I would have placed it on my head and eyes and obeyed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19720
Abdullah bin Wahb (may Allah have mercy on him) said that Imam Malik (may Allah have mercy on him) was asked about selling the clay which people eat, and he said: I do not like that something which harms people in their religion and their worldly life should be sold. Allah says: "They ask you what has been made lawful for them. Say: Good things have been made lawful for you." [Surah al-Ma'idah: 4]
Imam Malik (may Allah have mercy on him) said: I hold that the market supervisor should prevent them from selling it and forbid it. And Imam Malik (may Allah have mercy on him) said: It is also a kind of foolishness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19721